1. Field of the Invention
This invention relates to improved screening apparatus, in particular to sieves which combine a screen with a support.
2. Description of the Prior Art
Screens are used to separate and size different rocks, stones, gravel, water slurries and similar aggregate mixtures. Two characteristics are of major important in screens. One is the open area of the screen versus the total screen area, the larger the open area versus the total area the more efficient the screening process. The second is the wear life of the screen since a short life requires frequent screen changes which incurs both the screen and screen replacement costs. Both of these characteristics are economically important in the screening process.
Screens were originally formed only from woven wire. These screens were made of steel, stainless steel or spring steel wire. While wire screens have the largest open area versus the total screen area of about 50 to 55% of any screen, they also have the highest wear rate. To counter this high wear rate about 15 years ago screens were introduced made of polyurethane which greatly reduced the screen wear rate. While polyurethane screens greatly reduce the screen wear rate, they are considerably thicker than wire which reduces the amount of open area versus the total screen area to only 40 to 45 percent of the total area. This proportionally reduces the screen efficiency. Further, because of the lesser strength of polyurethane, cable rope under tension were incorporated into the screens to provide additional screen strength. This further reduces the open area and efficiency of the screens. Cables created another problem in that the cable had to remain under considerable tension to provide proper screen support. In manufacture, cables under tension are placed in an upwardly open mold which provided the frame shape, and mixed polyurethane plastic is poured around the cables until the frame is filled. The plastic is allowed to cure and the screens are then removed from the molds. After removal quite often the plastic will not hold the cables under the desired tension because of the inherent low friction between the plastic and cables.
Whenever polyurethane screens are substituted for wire screens, the structure which supported the wire screens must also be retrofitted to support polyurethane screens. The trade-offs between the relative advantages and disadvantages between wire and polyurethane screens have resulted in the industry currently using about 20% polyurethane screens and 80% wire screens.
It would be desirable if the wear characteristics of polyurethane could be obtained without the disadvantage of the reduction in open area versus open screen area, without the necessity for the use of cables to provide additional strength, and without having to retrofit the screen apparatus.